The CSG data are reorganized so that they are
in the form of common midpoint gathers, where any
trace in a CMG has the same source-receiver midpoint as any other trace
in the CMG.

A CMG collected along the Oquirrh Fault, Utah is shown in Figure 1.9.
Here the
midpoint xm and half-offset xh coordinates in a CMG are
related to the source xs and geophone xg coordinates in
a CSG by

xm= (xg+xs)/2

;

xh=(xg-xs)/2 .

(1.2)

The beauty of the CMG
is that each trace contains reflection energy that
sampled the same part of the reflector as the other traces in the CMG.
As an example, the R3 reflection events along
the R3 hyperbola in Figure
all emanated from apex of deepest raypath shown in this figure.
This redundancy will be exploited (as discussed later)
by stacking these redundant
reflections together to increase the S/N ratio of the
seismic record.
In contrast,
the R2 reflections in the CSG in Figure 1.8
emanate from different
parts of this reflector and so do not redundantly sample the R2
reflector.

Figure 1.9:
Similar to previous figure, except 1). traces in CSG's
are reorganized into a CMG, and 2).
the above data were collected along Oquirrh
Fault, Utah. Note, each ray shown above is connected with a source-receiver
pair, and all such pairs share the common midpoint location denoted
by the filled square box.
The thick hyperbolic lines describe the moveout curves for primary reflections.

Figure 1.10 depicts the rays and traces associated
with CSG's and CMG's, and the html movie shows
how these data were created.
Note, all of the common midpoint (CMP) rays
in the second to the bottom graph in Figure 1.10
emanate from the same reflection point.

The number of traces in a CMP gather defines the foldof the data. For example, the total number
of traces in Figure 1.9 define the fold number.
Large fold data means we have redundantly sampled
a subsurface reflection point many
times, so that after stacking (explained below) we will most
likely have a stacked trace with a good S/N ratio.
From the html movie,
note the fold of a CMG decreases as the midpoint position
approaches the end of the recording aperture.

Figure 1.10:
(Top 2 graphs) CSG's and associated rays. (Bottom
2 graphs) CMG seismograms and rays associated with
a common midpoint in the 2-layer model.
Check out the html movie to see how
the CSG's were generated and reorganized into CMG's.

The next step in order to realize the goal of ideal ZO data is to
apply time shifts
(i.e., Normal Moveout (NMO) corrections to be discussed in the next
section) to the traces in a CMP gather
to correct them to the zero-offset reflection time.
These corrected traces in the CMG
are then added together (i.e., stacked)
to produce a stacked trace with a large S/N ratio.
Presumably, most of the coherent noise can be
eliminated by this stacking process
because the time shifts only aligned the reflections with one another so that
only the primary reflections coherently added together after stacking.